1. Field of the Invention
The present invention relates to an accelerating system and method for a stepper motor, and in particular to an accelerating system and method using a safety accelerating curve which is similar to the characteristic curve of the stepper motor, to accelerate a stepper motor.
2. Prior Art
The conventional stepper motor is controlled by a micro controller. After the micro controller outputting a control signal, the control signal would be amplified by a driver, and then control the operation of the stepper motor. The control signal is usually like pulse signal. The frequency of the control signal is proportional to the rotational speed of the stepper motor. For example, 200 pulses per second could make the stepper motor to rotate a circle. That means that the higher pulse per second generated by the controller would make the stepper motor rotate in a higher rotational speed. But there is an inverse exponent ratio between the rotational speed and the torque of the stepper motor. Namely the higher rotational speed would generate the smaller torque. So, it must be very careful to accelerate the stepper motor to prevent the stepper motor from “out-of-step” or “out-of-speed”.
Referring to FIG. 1, which is a graph of a characteristic curve of a conventional stepper motor. The x-coordinate in the FIG. 1 shows the pulse per second received by the stepper motor. The y-coordinate in the FIG. 1 shows the output torque of stepper motor. The different characteristic between the stepper motor and the normal motor is that the stepper motor has two characteristic curves. The description of the two characteristic curves A and B shown in FIG. 1 is as below:    A: Pull-In Torque Curve            the pull-in torque curve shows the maximum torque the stepper motor which can synchronously start or stop following the control signals. Therefore, in the region below the pull-in torque, the stepper motor could be real-time started, stopped, positive and reverse rotated as the control signal requested. This region is always called start-stop region.            B: Pull-Out Torque Curve            the pull-out torque curve shows the maximum torque for the stepper motor which can synchronously operate following the control signal but can not immediately start or stop following the control signal. Between the pull-out torque curve and the pull-in torque curve, the stepper motor operates with the control signal but can't be started or stopped immediately. Beyond the pull-out torque curve, the stepper motor could not be operated. The region between the pull-out torque curve and the pull-in torque curve is called slew region. The stepper motor should be started or stopped in the start-stop region and then could be operated in the slew region. Otherwise, the stepper would be out-of-step.        
“Out-of-speed” means the rotational speed of the rotor in the stepper motor can't follow the rotational speed of the stator magnetic field. This would cause the stepper motor stop rotating. “Out-of-speed” would happen in every kind of motor. In the normal motor, “Out-of-speed” will cause the coil burning. Relatively, in the stepper motor, “Out-of-speed” only stops the stepper motor and don't cause the coil burning, even the exciter coil still exciting the magnetic fields, due to the stepper motor is controlled by the pulse signal.
“Out-of-step” means the stepper motor would slide out when the decreasing output torque can not afford the load of the stepper motor. Because the output torque is inverse proportional to the rotational speed, the rotational speed of the stepper motor raised in a short time will cause the output torque decreasing immediately.
The conventional stepper motor mostly are applied in the low speed region, so the linear accelerating method is usually used to accelerate the conventional stepper motor. Referring to FIG. 2, which shows the conventional accelerating curve adapted in the conventional stepper motor. The x-coordinate in the FIG. 2 shows the pulses per second received by the stepper motor. The y-coordinate in the FIG. 2 shows the output torque of stepper motor. The linear accelerating method can be applied when the conventional stepper motor is operated in the low speed region because the output torque still remains in the start-stop region and not cause any trouble. But when the stepper motor is accelerated with the rate in the high speed region, the accelerating curve will enter into the slew region and finally exceed the pull-out torque curve. In the conventional technology the pull-out torque curve is always supposed to be linear, but the accelerating line according to the linear accelerating method would easily cross the pull-out torque curve and then result the stepper motor having the trouble of “out-of-speed” (as the line C shown in FIG. 2). Moreover, because in the linear accelerating method the characteristic of the stepper motor is not considered, so the stepper motor can not guarantee to offer the desired output torque according to the linear accelerating method. Under this condition, the stepper motor can't afford the load and will be trapped in “out-of-step”.